1. Field of the Invention
The present invention relates to integrated circuit devices, and, in particular, to ring oscillator circuits for such devices.
2. Description of the Related Art
Ring oscillators have been used as a development vehicle for a variety of purposes, from process development to SPICE model validation to reliability qualification. See for example, Winnerl et al., International Conference on Microelectronic Test Structure, 1988. In a typical reliability qualification test circuit, a way to generate an on-chip AC signal is often desired. Ring oscillator (RO) circuits have been used to provide an on-chip digital waveform generator to stress/test metal lines for electromigration, to test oxide capacitors for gate oxide integrity, for hot-carrier stressing in devices, and for examining other process reliability related issues.
Two parameters of interest of a ring oscillator output are the frequency of the signal (fosc) and the duty cycle (DC) of the waveform. To acquire multiple frequencies in an on-chip test circuit, one would typically have to design many different ROs with different numbers of stages to vary the frequency. To overcome this need for multiplicity, Snyder et al. proposed the current-starved inverter approach as a way to implement a frequency modulation capability and thereby achieve continuous frequency range from a single RO. See for example, Snyder et al., International Conference on Microelectronic Test Structure, 1994. Although beneficial, the current-starved inverter design still lacks an ability to control variability of the waveform duty cycle, and an ability to implement this capability easily.
Thus, it can be seen that modern ring oscillator techniques impose duty cycle and oscillation frequency limits upon IC-based reliability test circuitry, and hinder the use of these testers in many applications.
Therefore, there is an unresolved need for an improved ring oscillator technique that can easily modulate and control duty cycle and oscillation frequency for IC-based reliability test circuitry.
A ring oscillator technique is described that can easily modulate and control duty cycle and oscillation frequency for IC-based reliability test circuitry.
A control circuit for a ring oscillator uses an integrator and a comparator. The integrator receives a square wave signal as an input and provides a triangular wave signal as an output. The comparator generates a second square wave signal by comparing the triangular wave signal and a reference voltage. Duty cycle for the ring oscillator can be modulated by varying the reference voltage.
For one embodiment of the control circuit, signal frequency of the ring oscillator can be modulated by varying biasing current for the comparator to set slew rate of the comparator.
For one embodiment of the control circuit, a resistor and a capacitor form a simple RC circuit for the integrator. For an alternative embodiment of the control circuit, a resistor and an amplifier with a capacitor in a negative feedback loop form an active RC circuit for the integrator.
For one embodiment of the control circuit, the comparator is an inverting comparator.
A ring oscillator employing the control circuit can be formed using a plurality of inverters coupled in series with the control circuit.
A test system can be formed by coupling an output buffer and the ring oscillator to a device-under-test and monitoring the ring oscillator frequency and device characteristics.